This graph shows the relationship between the temperature and brightness of a sampling of stars in the globular cluster M4. The line of dots at upper right represents main-sequence stars, those whose cores are fusing hydrogen. Normal stars like our Sun fall in the middle of this line.

The white dwarfs are not part of the main-sequence stars because they have used up their hydrogen fuel. These dying stars are fainter and yet hotter than their main-sequence relatives. NASA's Hubble Space Telescope hunted for the faintest and coolest of the white dwarfs [bottom of white dwarf population.]